Over the past decades, improving energy conversion efficiency of InGaN/GaN blue light-emitting diodes (LEDs) has been persistently pursued. The continuous improvement of the energy efficiency of the InGaN/GaN LEDs mainly originated from developments in wafer growth techniques, packaging technologies, and nanophotonics. The major bottleneck toward achieving high efficiency LEDs is the low hole injection rate from the p-type hole transport layer (p-GaN:Mg) due to insufficient activation of the p-type dopant. The poor hole injection leads to a significant imbalance between the number of electrons and holes, leading to poor electron-hole recombination rates and, thus, low efficiency at high current densities. A number of research efforts have been geared toward making a breakthrough in this issue, including formation of a surface polarization layer, electron blocking layer, tunnelling layer, graded barrier structure, and nanopatterning. While these approaches helped improve energy conversion efficiency, there still exist issues such as the high cost for wafer regrowth and non-uniform surface patterning over a full-wafer area, in addition to strains induced by different thermal expansion coefficients and by different lattice constants between substrates and epitaxial layers.
The InGaN/GaN LEDs grown on a c-plane sapphire substrate possess a piezoelectric polarization field induced by the lattice mismatch between the InGaN and GaN layers. Additionally, the Wurtzite crystal structure of GaN generates a spontaneous polarization field in the LEDs, which consequently forms tiled energy bands within the InGaN/GaN multi-quantum wells (MQWs), leading to a reduced spatial distribution of electron and hole wave functions and, thus, reduced radiative recombination rates (i.e., quantum-confined Stark effect (QCSE)). Several methods were attempted to suppress the QCSE in LEDs, such as adaptation of nonpolar or semipolar substrates, polar MQWs with large wave function overlap design, substrate variation, polarization-matched epi-layer, top surface modification, and 1-dimensional vertical structure array. These methods require complicated device design, special/expensive substrates, and skillful epitaxy techniques. Various processes for LED top surface modification were also attempted including surface texturing, less-strained layer growth, and thin layer deposition. Although these surface modification approaches are simpler than other epitaxial methods, the cost associated with these approaches may still be high.